1. Field of the Invention
The present invention relates to a compact liquid crystal panel and a liquid crystal projector which employs the liquid crystal panel so as to project onto a screen at an enlarged scale, an image displayed on the liquid crystal panel.
2. Description of the Prior Art
Liquid crystal panels have a number of features such as being light weight and thin and thus, are under intensive research and development. However, the liquid crystal panels have also many problems. For example, it is difficult to enlarge image planes of the liquid crystal panels. Thus, in recent years, a liquid crystal projector that has increasingly attracted much attention is one in which an image displayed on a compact liquid crystal panel is projected onto a large screen at an enlarged scale by a projection lens or the like such that a large image is displayed on the screen. Liquid crystal projectors commercially available at present employ a twisted nematic (TN) liquid crystal panel based on optical rotation of liquid crystal.
Initially, a liquid crystal panel will be generally described. The TN liquid crystal panel as one example of known liquid crystal panels requires polarizers at its input and output sides, respectively so as to modulate light. The operation of the TN liquid crystal panel will be described briefly. When a light ray has passed through the input polarizer, the light ray is turned into unidirectional polarized light such that the polarized light is incident upon the liquid crystal panel. When the liquid crystal panel is in the OFF state the, direction of polarization of the incident light ray is rotated through 90.degree. by optical rotation of liquid crystal molecules. On the contrary, when the liquid crystal panel is in the ON state, optical rotation of the liquid crystal molecules is eliminated and thus, the direction of polarization of the incident light ray does not change. Hence, if the directions of polarization of the input and output polarizers intersect with each other at right angles, the incident light ray is transmitted through the liquid crystal panel and intercepted when the liquid crystal panel is in the OFF state and ON state, respectively. Meanwhile, if the directions of polarization of the two polarizers are parallel to each other, the result is opposite, namely, the incident light ray is intercepted and transmitted through the liquid crystal panel when the liquid crystal panel is in the OFF state and ON state, respectively. The TN liquid crystal panel modulates light by turning on and off as described above so as to display an image.
Hereinbelow, a known liquid crystal projector is described. The known liquid crystal projector includes a light emitting means such as a converging optical system or the like, an ultraviolet cutting mirror for transmitting ultraviolet light therethrough, a blue dichroic mirror for reflecting blue light (referred to as a "BDM", hereinbelow), a green dichroic mirror for reflecting green light (referred to as a "GDM", hereinbelow), a red dichroic mirror for reflecting red light (referred to as a "RDM", hereinbelow), optical elements such as projection lens systems, etc. used for the blue light, green light and red light, respectively, three pairs of input and output polarizers used for the blue light, green light and red light, respectively and conventional transmission type TN liquid crystal panels for the respective pairs of the input and output polarizers.
The known liquid crystal projector operates as follows. Initially, when white light is emitted from the converging optical system, blue light is reflected from the white light by the BDM so as to be incident upon the corresponding input polarizer. From light which has been transmitted through the BDM, green light and red light are, respectively, reflected by the GDM and the RDM so as to be incident upon the corresponding input polarizers. Only the light oscillating in one direction for each color is transmitted through each of the input polarizers so as to be turned into s-polarized light or p-polarized light and the light oscillating in a direction orthogonal to the one direction is absorbed by each of the input polarizers. The s-polarized light or the p-polarized light irradiate to each liquid crystal panel. Each liquid crystal panel modulates the transmitted light by an image signal. The modulated light is transmitted through each output polarizer in accordance with its modulation degree and is incident upon each projection lens system so as to be projected onto a screen at an enlarged scale.
As is apparent from the foregoing, if the liquid crystal panel employs TN liquid crystals, linearly polarized light is required to be incident upon the liquid crystal panel. Therefore, the input and output polarizers should be provided forwards of and rearwards of the liquid crystal panel, respectively. Theoretically, the polarizers absorb 50% or more of light. Therefore, such a problem exists that when light is projected onto a screen at an enlarged scale, only an image of low luminance is obtained. Furthermore, the light absorbed by the polarizers is turned into heat, thus resulting in a deterioration of the reliability of the polarizers and the liquid crystal panel.
As one example of a prior art liquid crystal panel employing no polarizer, U.S. Pat. No. 4,435,047 proposes a polymer dispersion liquid crystal element in which liquid crystal droplets are formed in a polymer matrix such that a changeover between light scattering and light transmission is performed by impressing a voltage. However, in this prior art polymer dispersion liquid crystal element, since light scattering is insufficient, it is impossible to obtain an image of both high luminance and high contrast.
Meanwhile, U.S. Pat. No. 4,389,096 or U.S. Pat. No. 4,729,640 proposes an element in which nematic liquid crystals and a diffraction grating are combined with each other. However, liquid crystal is a uniaxial birefringent crystal. Thus, if material having an isotropic refractive index is used for the diffraction grating, the element operates based on changes of polarization and characteristics of the diffraction grating change according to the direction of polarization. Namely, light oscillated in a specific direction is diffracted according to a difference in refractive index between the diffraction grating and liquid crystal. However, light oscillated in a direction orthogonal to the above specific direction is not modulated at all because of no difference in refractive index between the diffraction grating and liquid crystal. Namely, a mere 50% of natural light can be modulated.
In order to solve the above-mentioned problems of TN liquid crystals, the present invention employs a polymer dispersion liquid crystal material. In a liquid crystal panel employing polymer dispersion liquid crystal, since no polarizer is used, its optical efficiency can be quite high. Furthermore, by forming the diffraction grating on a surface of a substrate, an image of high contrast is obtained.
Polymer dispersion liquid crystal is described briefly, hereinbelow. Polymer dispersion liquid crystal can be roughly divided into two types according to the state in which the liquid crystals are dispersed in the polymer. In one type, liquid crystal particles in the form of water drops are dispersed in polymer. In this type, the liquid crystal particles are present in a discontinuous state in the polymer. Hereinbelow, this type of the liquid crystal is referred to as "PDLC" and a liquid crystal panel employing PDLC is referred to as a "PD liquid crystal panel". In the other type, a network of polymer is stretched throughout a liquid crystal layer as if the liquid crystal were absorbed by a sponge. In this type, the liquid crystal is not in the form of water drops but is present continuously in the polymer. Hereinbelow, this type of the liquid crystal is referred to as "PNLC" and a liquid crystal panel employing the PNLC is referred to as a "PN liquid crystal panel". When an image is displayed by using one of the above-mentioned two kinds of liquid crystal panels, light scattering and light transmission are controlled by an electric field.
The PD liquid crystal panel operates based on the fact that there is a difference in refractive index between polymer and liquid crystal in a direction of alignment of the liquid crystal. When no voltage is applied to the liquid crystal panel, the liquid crystal particles in the form of water drops are oriented in irregular directions. In this state, a difference in refractive index between the polymer and the liquid crystal is produced and thus, incident light is scattered. If a voltage is applied to the liquid crystal panel at this time, the liquid crystal particles are aligned in one direction. If a refractive index of the liquid crystal at the time when the liquid crystal particles are aligned in one direction is preliminarily set to a refractive index of the polymer, incident light is transmitted through the liquid crystal panel without being scattered.
On the other hand, the PN liquid crystal panel is based on the irregularity of the orientation of liquid crystal molecules. In an irregular state of orientation, namely, when no voltage is applied to the liquid crystal panel, incident light is scattered. On the contrary, when the liquid crystal molecules are aligned in one direction by applying a voltage to the liquid crystal panel, light is transmitted through the liquid crystal panel. Meanwhile, it should be noted that the operation of the PD liquid crystal panel and the PN liquid crystal panel have been described only in general terms.
The present invention is not limited in its application to one of the PD liquid crystal panel and the PN liquid crystal panel. However, in order to facilitate the description of the present invention, reference will only be made to the PD liquid crystal panel. Meanwhile, the PD liquid crystal panel and the PN liquid crystal panel will be generically referred to as a "polymer dispersion liquid crystal panel".
Thermoplastic resin or thermosetting resin may be used as a polymer matrix of the polymer dispersion liquid crystal layer as long as it is basically transparent. However, ultraviolet-curing resin, which is most convenient and has excellent performance, is generally used because a conventional production method of the TN mode liquid crystal panel can be applied to the ultraviolet-curing resin without any modification. In order to produce a liquid crystal panel, the following method has usually been employed. Namely, predetermined electrode patterns are preliminarily formed on upper and lower substrates, respectively, and the upper and lower substrates are piled on each other such that electrodes on the upper and lower substrates confront each other. At this time, in a state where a spacer having a predetermined uniform particle size is interposed between the upper and lower substrates so as to secure a clearance between the upper and lower substrates, the substrates are secured by a sealing compound made of epoxy resin and thus, a hollow cell is obtained. Then, liquid crystal is injected into the hollow cell.
In order to produce a polymer dispersion type of liquid crystal panel by applying this known production method, ultraviolet curing resin, for example, acrylic resin, may be used as the polymer matrix. Acrylic resin exists as a precursor having a relatively low viscosity, e.g., a monomer or oligomer before injection and acrylic resin blended with liquid crystal has sufficient fluidity to be injected into the hollow cell at ordinary temperature. Therefore, if a method is adopted in which the polymer dispersion liquid crystal is injected into the hollow cell using a method known in the production of liquid crystal panels and then, the cell is irradiated so as to cause a curing reaction, the polymer dispersion type liquid crystal panel can be produced easily.
Meanwhile, by irradiating the panel after injection, a polymerization reaction is caused only in the resin so as to change the resin into a polymer and thus, only the liquid crystal is phase-separated from the polymer. If the amount of liquid crystal is small in comparison with that of the resin, independent liquid crystal particles in the form of water drops are formed. On the other hand, if the amount of the liquid crystal is large as compared with that of the resin, a polymer matrix is present in the liquid crystal in the form of particles or a network such that the liquid crystal forms a continuous layer. At this time, unless the diameter of the liquid crystal particles or the diameter of pores of the polymer network is generally uniform and ranges from 0.1 .mu.m to several .mu.m, light scattering performance is poor and thus, contrast is not raised. Therefore, the resin should be cured in a relatively short period. To this end, ultraviolet curing resin is desirable as the resin.
As described above, since the polarizers are not required to be provided in the polymer dispersion liquid crystal panel, the polymer dispersion liquid crystal panel has high optical efficiency so as to display an image of quite high luminance. However, if the above-mentioned polymer dispersion liquid crystal is used for the liquid crystal panel, the following two problems arise. One problem is that the polymer dispersion liquid crystal layer is separated from an opposed electrode or a pixel electrode. This phenomenon occurs because close contact between the polymer dispersion liquid crystal layer and the electrodes made of indium-tin-oxide (ITO) or the like is low. In a liquid crystal projection type television set, heat of 50.degree. to 60.degree. C. is applied to the liquid crystal panel during the "ON" state of a lamp acting as a light source. On the contrary, the liquid crystal panel is set to room temperature of 10.degree. to 30.degree. C. during the "OFF" stale of the lamp. Accordingly, by turning on and off the light source of the liquid crystal projection type television set, the liquid crystal panel is subjected to severe thermal shock. The above mentioned separation is caused by this thermal shock, etc.
The other problem is that light scattering characteristics are poor. In order for this polymer dispersion type liquid crystal panel to be practical, the liquid crystal panel should not only be driven at low voltage but should display an image of sufficient contrast. In order to obtain an image of remarkably excellent quality, it is preferable that contrast ratio is not less than 30:1 for direct vision type and not less than 100:1 for projection type. In order to increase the contrast ratio, light scattering characteristics should be improved. Perfect diffusion may be considered as one goal of light scattering action. As described in Proceedings of SID, p.138 (1977) by Dewey, the contrast ratio CR in perfect diffusion can be calculated from the following equation.
CR=1/sin.sup.2 .sigma.
In the above equation, .sigma. denotes a converging angle (half angle). Up to the present, light scattering characteristics of the polymer dispersion liquid crystal panel are rather inferior to those in a perfect diffusion state which is ideal light scattering state.
When a currently available polymer dispersion liquid crystal panel is used as a projection type display, the f-number of a concave mirror converging optical system employing a metal halide lamp use and a projection optical system matched with the concave mirror converging optical system ranges from 4 to 5 and thus, the contrast ratio is as insufficient as 35:1.